1. Field of the Invention:
The present invention is directed to the field of optical windows and more specifically to those windows which are able to transmit high energy radiation.
2. Description of the Prior Art:
High energy laser (HEL) weapons systems are discussed as utilizing sources, such as DF and CO.sub.2 lasers to generate beams having power densities on the order of 30 w/cm.sup.2 and greater and having diameters in the range of from 0.4 to 4 meters. Due to the large size of the source required to generate such a large beam, it is most practical to provide a beam pointing subsystem external to the source in order to expand and control the direction of the beam from the weapon carrier (aircraft, ship, satellite or ground vehicle) towards a designated target.
Such a beam pointing subsystem must, by necessity, be lightweight, fast and accurate. As part of the beam pointing subsystem, a protective housing must also be provided as a barrier between the outside environment and a controlled internal environment. Such a housing also prevents direct damage to the beam pointing optics and should also reduce the amount of dust and other contaminants that would degrade the beam pointing optics.
The protective housing must also provide a means for allowing unaltered transmission of the beam from the beam pointing subsystem to a designated remote target. Several alternative systems have been proposed to provide a beam transmission port, including solid state windows, aerodynamic windows and open-port (positive pressure) systems.
A solid state window is most highly desired, due to its inherent ability to maintain the internally mounted optics free from contamination and allow for a controlled inner environment to be maintained within the housing. However, very few materials exist which possess suitable characteristics. Due to the very large size of the transmission port aperture, and the large amount of energy contained in the HEL beam, solid state rigid materials provide a very limited solution. Solid material windows of such a large area must necessarily be thick in order to provide the required strength to be self-supporting. This, in turn, results in excessive losses, and distortions; and introduces the possibility of fracture due to the energy absorbed from the beam. The additional mass, due to such a heavy window, degrades performance of the pointing system. In addition, fabrication techniques applicable to optical quality windows of this size are not presently available. Segmenting such window material results in additional losses due to the supporting struts which must necessarily be inserted between segments. Furthermore, an additional optical degradation may occur due to possible segment misalignments.
The use of aerodynamic window technology is shown in U.S. Pat. No. 3,902,136 in which a rotating bladerow is employed to maintain the internal high pressure gas within a laser and preventing it from passing to the surroundings that are at a lower pressure. Such a system, of course, allows for the elmination of any solid material in the aperture of the laser. On the other hand, the system requires a sophisticated mechanism to provide the drive for the rotating bladerow that, in the case of the large aperture discussed above, may be difficult to implement in view of the size and weight restrictions that must necessarily be considered.
An open-port (positive pressure) system also eliminates the solid window mentioned above but, at the same time, requires a large supply of uncontaminated air to be pumped into the housing. For an aperture the size of that discussed above, a considerable volume of air would be required to be pumped into the housing and would not allow for one to utilize an inert atmosphere. In both cases above, required pressures and resulting flow-rates would, of necessity, have to exceed outside wind conditions to provide the required protection. As a result, the high levels of turbulence thus induced significantly degrade the beam traversing that region.